EMPA20110483 5 Flame retardant functional textiles S. GAAN , V. SALIMOVA , P. RUPPER , A. RITTER and H. SCHMID , Swiss Federal Laboratories for Materials Testing and Research (EMPA), Switzerland Abstract : This chapter is a brief review of research and developments in the fi eld of fl ame retardant textiles. The review focuses mainly on the currently available fl ame retardant solutions for different kinds of textiles. It gives insights into the general mode of action of fl ame retardants, types of fl ame retardant fi bers, fl ame retardant additives for fi bers and surface treatments and standard test methods for fl ame retardant textiles. The existing commercial fl ame retardant treatments for textiles are over 50 years old and some of them have created environmental concerns. Over the past few decades there have been considerable developments in inherently fl ame retardant synthetic fi bers but effective and environmentally friendly fl ame retardant coatings or fi nishing solutions for textiles still elude us. Key words : fl ame retardant, inherently fl ame retardant fi ber, textile, phosphorus, polymer. 5.1 Introduction Textiles are manufactured from fi ber forming natural polymers such as cel- lulose and protein and a wide variety of synthetic polymers such as poly- Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 esters, polyolefi n, polylactide, polyamides, polyaramids, polyetherketones, polyacrylonitrile and cellulose acetate. All of these polymers are suitable for use in textiles because of excellent fi ber-forming properties, but they share a common problem, namely that most of them are combustible under normal environmental conditions and pose serious fi re hazards in case of fi re accidents. These organic polymers are a rich source of hydrocarbons and thus they are an excellent source of fuel during the burning process. The term ‘fl ame retardant textiles’ usually refers to textiles or textile-based materials that inhibit or resist the spread of fi re. The hazards and risks asso- ciated with textiles are well documented elsewhere (Horrocks, 2001 ) and will not be discussed in this review. Flame retardant textiles fi nd use in vari- ous areas such as (a) manufacturing of uniforms for fi re fi ghters (Prezant et al ., 1999 ), military/police personnel (Adanur and Tewari, 1997 ; Duran 98 © Woodhead Publishing Limited, 2011 Flame retardant functional textiles 99 et al ., 2007 ; Rodie, 2008 ) and industrial workers; (b) high-performance sports applications (Stegmaier et al ., 2005 ); (c) upholstery for home furnish- ing (Kamath et al ., 2009 ), offi ce/commercial infrastructure and transpor- tation (Flambard et al ., 2005 ); and (d) sleepwear for children and elderly people (Horridge and Timmons, 1979 ; Horrocks et al ., 2004 ). Flame retar- dant (FR) textile materials help save lives, prevent injuries and property losses and protect the envir onment by helping to prevent fi res from starting and limiting fi re damage. Flame retardant textiles can be constructed from inherently fl ame retardant fi bers such as Nomex, Kevlar, wool, Trevira CS, modacrylics, melamine fi bers and polyvinyl chloride (Marsden, 1991 ; Weil and Levchik, 2008 ). Alternatively, fl ame retardant textiles can also be manu- factured either by surface treatments with fl ame retardant compounds with various chemistries or by incorporating fi re retardant additives in polymer bulk before fi ber spinning. 5.2 Factors affecting flammability and thermal behavior of textile fibers and fabrics The fl ammability of a textile material is a very complex phenomenon and depends on many factors such as the polymer itself, construction of the textile (i.e. weave/knit of fabric, yarn construction), weight/unit volume, additives in the fi ber, the type of chemical treatments and the test condi- tions (Nair, 2000 ; Ozcan et al ., 2003 ). All textiles can burn provided they are exposed to the right conditions of fl ame, heat, oxygen concentration and time of exposure to fl ame. 5.2.1 Fabric construction In fl ammability tests, the inherent resistance of polymers to burning was the major factor affecting fabric performance with weight, weave and thickness Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 being secondary. Fabric structure, weight and thickness were more impor- tant when measuring thermal protective capability. A combination of fi ber non-fl ammability and optimum fabric structure should be used to achieve maximum non-fl ammability with the lightest weight for the specifi c fabric application (Barker and Brewster, 1982 ; Ross and Stanton, 1973 ). Availability of oxygen is a critical factor in determining the fl ammabil- ity of a fabric. The construction of a fabric can play an important role as it determines the amount of air present, the active surface area and the fl ow of air through the fabric (Hendrix et al ., 1972 ). Fabrics with open constructions may be more combustible and the fl ame propagation will also be faster. A fabric with raised surface (e.g. fl eece-style fabrics, fl annelettes and terry toweling) and containing fi ber protruding out to the surface needs special care during processing and use. The fl ammability hazard with raised fi ber © Woodhead Publishing Limited, 2011 100 Functional textiles for improved performance, protection and health surface fabrics involves the phenomenon called ‘surface fl ash’ whereby a fl ame can travel rapidly over the fabric surface, singeing the fi ber ends. This fl ash, in itself, may not be dangerous unless the intensity of the fl ame is suffi - cient to ignite the base fabric. In testing, this is known as timed surface fl ash with base burn. The twist of a yarn may also affect the porosity of the fab- ric. A tightly woven fabric with higher density, constructed from yarns with high twist will provide better fl ame protection. Fabric blends can also play a crucial role in defi ning the fl ammability of fabrics. Blends of fabric made from two different fi brous polymers may show fl ammability characteristics quite different from what is exhibited by each component of blend indepen- dently. For example, fabrics made from polyester are less fl ammable than cotton fabrics, but polyester/cotton blended fabrics burn more rapidly. This is because of the ‘scaffolding’ effect, where the charred cotton in the blend acts as a support for the polyester fi bers. The melting polyester in the blend does not drip away as it may do in 100% polyester fabrics, and continues to burn (wicking effect). The fl ammability and thermal shrinkage are relatively independent of the fabric weight and weave design and primarily depen- dent on the thermal stability of component fi bers (Minister of Health, 2009 ; Stepniczka and Dipietro, 1971 ). 5.2.2 Type of textile fi ber The fi bers used in textiles may be broadly classifi ed based on natural poly- mers or synthetic polymers. Cotton, wool, silk and fl ax are some of the major natural textile fi bers while viscose, Tencel™ and cellulose acetate are derived from natural polymers. The synthetic textile fi bers owe their origin to several polymer classes such as polyester, polyamide, polyacrylonitrile, polyolefi ns, polylactide, polyaramid, melamine based, polyurethane, etc. The fl ammability properties of the textile fi bers differ and can be grouped based on their burning characteristics into the following (Minister of Health, Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 2009 ): • Readily fl ammable: these fi bers ignite readily and burn rapidly, leaving a light ash residue (e.g. cotton acetate, triacetate, rayon and ramie). • Moderately fl ammable: these fi bers are more diffi cult to ignite. The syn- thetics tend to melt and drip, sometimes self-extinguishing upon removal of the ignition source (e.g. acrylic, nylon, polyester, olefi n and silk). • Relatively non-fl ammable: in general, these fi bers will not support com- bustion after removal of the ignition source (e.g. wool, modacrylic, vinyon and aramids). Burning behavior of fi bers is related to the thermal transition temperatures and thermodynamic parameters such as glass transition temperature ( Tg ) © Woodhead Publishing Limited, 2011 Flame retardant functional textiles 101 and melting point ( Tm ). Chemically related transitions such as temperature for onset of pyrolysis ( Tp ) and combustion ( Tc ) are also very important. The lower the Tc values and the higher the fl ame temperature, the higher is the fl ammability of a fi ber. The limiting oxygen index (LOI) of fi bers is also an important property which determines the fl ammability of the fi bers. Fibers with LOI values greater that 21% burn slowly and LOI values of 26–28% are suffi cient to pass small burning tests. The thermal and fl am- mability properties of fi bers are discussed in detail elsewhere (Horrocks, 2001 ; Stegmaier et al ., 2005 ). All cellulose-based fi bers have very low LOI values (<19%) and are thus easy to burn. Among the natural fi bers wool has relatively high LOI value (approx. 25%) and is a common component in manufacturing fl ame retardant clothing for fi refi ghters in some countries such as Australia. Another criterion which determines the fl ammability of fi bers is the heat release rates of fi bers (i.e. the speed at which the heat is released during thermal decomposition) (Horrocks, 2001 ; Yang et al ., 2009 ). Peak heat release rates of some common fi bers like cotton, rayon, cellu- lose acetate, silk, nylon, polyester, polypropylene, acrylic fi bers, Nomex and Kevlar have been recently measured using micro-scale combustion calorim- etry.